96 Journal of Agricultural Research voi. xxii. no. , 



draw tube of the microscope so that the field would be of the greatest 

 possible diameter without losing definition, as the greater the diameter 

 of the field the less the increment of error in the total counts. 



The thickness of the section is controlled by adjusting the microtome 

 to cut sections of a desired and known thickness. If all the adjustments 

 on the microtome are firm and a sharp knife is used, sections can be cut 

 of uniform thickness with surprising accuracy. The thickness of the 

 sections can also be remeasured with the fine adjustment screw on the 

 microscope. Although not perfect, this method of measurement serves 

 as a check upon the accuracy of the sectioning. The measurement is 

 accomplished by focusing with the graduated fine adjustment screw on 

 both the upper and lower surfaces of the section and noting the differ- 

 ences in the readings between the two levels. The difference can be 

 read in microns where graduations are given on the fine adjustment 

 screw. 



To convert the per-cubic-centimeter counts into numbers per gram, 

 the specific gravity of the cheese must be considered. As the specific 

 gravity of all samples has been assumed to be approximately i, the 

 counts are interchangeable. This assumption in regard to the specific 

 gravity is arbitrary, but the variations in the specific gravity of cheddar 

 cheese are so slight that the total count is not affected to any appreciable 

 degree. Accurate determinations did not seem practicable, as the speci- 

 fic gravity varies with the fat content and with the moisture and general 

 consistency of the cheese. 



With the measurements and adjustments used in this laboratory the 

 per-gram formula resolves itself into the following: 



r = 0.07 mm. (70 fx). 



a = .005 mm. (5 /x). 



6 = 1.0. 



1 ,000 



7—- X I = approximately i-; ,000,000. 



3.1416 X 0.0049 X 0.005 



APPLICATION OF THE METHOD 



It is evident that this microscopic technic is subject to the limitations 

 of any direct method of examination, many of which are unavoidable 

 and are due to mechanical limitations or to the human error, which 

 enters in when counts or estimates are made. 



QUALITATIVE EXAMINATION 



As previously stated, cheese has been examined microscopically by 

 many investigators. The possibility of error is not as great when samples 

 are examined to determine the types of organisms present as when 

 total count is made, which is true of any microscopic work. Our present 

 staining methods make possible a direct visualization of the microorgan- 



